The present invention relates generally to a hydraulic fluid pressure control system for an automobile automatic transmission, and, more particularly, relates to an improvement in such a hydraulic fluid pressure control system which is used for controlling an automatic transmission which is equipped with a lock-up clutch for locking up a fluid torque converter thereof.
There is known an automatic transmission for automobiles or other vehicles, which comprises a fluid torque converter, a lock-up clutch which, when actuated by supply of hydraulic fluid pressure thereto, directly connects the input and output members of the fluid torque converter, and a gear transmission mechanism equipped with a plurality of friction engagement means for providing a plurality of transmission speed stages. Such an automatic transmission is controlled by a hydraulic fluid pressure control system, which provides selective supply of actuating hydraulic fluid pressures for engaging or disengaging said plurality of friction engaging means, in order correctly to provide a desirable speed stage, according to the operating conditions of the automobile, such as its road speed and the torque being developed by the engine thereof.
In operation of an automatic transmission of this type, it is conventional for the lock-up clutch to be engaged only when the highest speed stage is being provided by the hydraulic fluid pressure control system, when the most commonly used transmission range ("D" range for example) is selected on the manual range selection means, during normal operating conditions; or, alternatively, when the automobile speed is greater than a certain specified value, irrespective of the transmission range selected by the manual range selection means.
However, disadvantages arise with these modes of operation. If the lock-up clutch is engaged only when the highest speed stage is attained in the transmission, the lock-up clutch cannot be effectively utilized when the automobile speed is being reduced by using engine braking by shifting down the selected range of the automatic transmission from this highest speed range to a lower speed range, such as "2" range, which is actually in fact a very desirable time to utilize the lock-up clutch. Further, if the lock-up clutch is always kept engaged when the automobile speed is greater than a certain specified value, irrespective of the speed stage attained, there also arises a disadvantage that, if shifting of the transmission speed stage provided by the automatic transmission occurs during this condition, the action of the fluid torque converter for cushioning the sudden jerking inevitably produced during such speed stage change will not be available, and accordingly the durability of the lock-up clutch, and of the automatic transmission as a whole, may be severely adversely affected because of transmission impact.